This invention relates to an x-ray tube anode target and, to a coating on an x-ray tube anode target. More particularly, this invention relates to an x-ray tube target including an amorphous carbon coating. Furthermore, the invention relates to a process for forming the coating.
Ordinarily, an x-ray beam generating device, referred to as an x-ray tube or target, includes dual electrodes in an evacuated chamber. One of the electrodes is a thermionic emitter cathode which is positioned in the tube in a spaced relationship to a target anode. Upon energization of an electrical circuit, the cathode is electrically heated to generate a stream or beam of electrons directed towards the x-ray target anode. The electron stream is appropriately focused as a thin beam of very high velocity electrons striking the target anode surface. The anode surface is usually constructed of a refractory metal so that the kinetic energy of the striking electrons against the target material is converted to electromagnetic waves of very high frequency, i.e., x-rays which radiate from the target and are collimated and focused for penetration into an object, usually for internal examination purposes. Well known primary refractory metals for the anode target surface area exposed to the impinging electron beam include tungsten, molybdenum, and their many alloys. The high velocity beam of electrons impinging the target surface generates extremely high and localized temperatures in the target structure accompanied by high internal stresses leading to deterioration and break down of the target structure. As a consequence, it has become routine to use a rotating disk shaped anode target, one side or face of which is exposed to the electron beam from the thermionic emitter cathode. By means of target rotation, the impinged region of the target is continuously changing to avoid localized heat concentration and stresses and to better distribute the heating effects throughout the structure. Accordingly, rotation of targets for improved heat dissipation has reached target speeds exceeding 10,000 RPM. Nonetheless, heating remains a major problem in x-ray anode target structures.
A target body is chosen from a material with a high heat storage capacity. Moreover, only about 1% of the energy of the impinging electron beams convert to x-rays with the remainder appearing as heat which must be rapidly dissipated from the target by means of heat radiation. One preferred material for the rotating disk-like anode target body is graphite which has a high heat storage capacity and which readily accepts bonding to a refractory metal layer such as the cathode electron beam impinging surface. Accordingly, significant technological efforts are expended towards improving heat dissipation from x-ray anode target surfaces.
Methods for dealing with heat stress are provided by various techniques, such as U.S. Pat. No. 4,939,762, herein incorporated by reference, wherein a tungsten-rhenium alloy is coated on a graphite body to protect the x-ray generating metal from any excessive thermal load. More particularly, to address the problem of peel away of the x-ray generating metal coating, the utilization of a metal interlayer between a tungsten containing x-ray generating metal coating and the graphite body is taught.
In U.S. Pat. No. 5,414,748, herein incorporated by reference, another method used to treat heat related problems associated with x-ray targets is disclosed. It is taught that the x-ray tube include a circular graphite body having a circular metal alloy target section disk concentrically bonded to the graphite body, the target section disk has a peripheral axial rim surface. A high heat emissive hafnium carbide coating is then deposited on the rim of the target section disk to improve heat emission.